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The Scent of Seduction

Orchids use a crafty blend of pheromones to lure pollen-laden male bees to their flowers, biologists report in tomorrow's Nature. The findings suggest that, like ad agencies the world over, plants have found that sex sells.

Plants use a variety of enticements to attract would-be pollinators. Their charms range from the allure of bright colors to the sweet scent of nectar, or even the rancid smell of rotting meat. Many plants rely on the amorous instincts of other species to help satisfy their own reproductive needs--their flowers imitating, for example, the color or shape of a female insect to attract males. Now, Florian Schiestl, an evolutionary biologist at the University of Vienna in Austria, has found that the flowers of the orchid Ophrys sphegodes mimic not only the look and feel of the female solitary bee Andrena nigroaenea, but also her sophisticated perfume.

Using a technique called gas chromatography, Schiestl and colleagues in Sweden and Germany separated the components of the female bee extract that sexually arouses males, then measured the electrical response of male antennae to each compound to determine which ones are active. The team found that the relative proportions of the compounds in the bee extract were remarkably similar to those in orchid flower extract. When they applied either the orchid extract, bee extract, or synthetic versions of either to decoy females, all the concoctions drove male bees wild. When the researchers tested the potency of extracts from other orchids of the same genus, and from the leaves of O. sphegodes--all of which contain the same active compounds, but in different proportions--they elicited no sexual response in the males.

The findings contradict earlier work, which had suggested that Ophrys scent was not well matched to the female bee. But Schiestl's work convinces Jocelyn Millar, who says it goes "a large step beyond" previous investigations by using male antennae to identify the active compounds. Millar, a chemical ecologist at the University of California, Riverside, expects to see "a cascade of studies" on what he calls an excellent example of the coevolution of insect-plant interaction.